cases: a novel low-cost ground-based dual-frequency gps
TRANSCRIPT
CASES: A Novel Low-Cost Ground-based Dual-
Frequency GPS Software Receiver and Space
Weather Monitor Geoff Crowley, Gary S. Bust, Adam Reynolds, Irfan Azeem, Rick Wilder, ASTRA, Boulder CO
Brady W. O’Hanlon, Mark L. Psiaki, Steven Powell, Cornell University, Ithaca NY
Todd E. Humphreys, Jahshan A. Bhatti, The University of Texas, Austin TX
BIOGRAPHIES
Geoff Crowley is President and Chief Scientist of Atmospheric & Space Technology Research Associates
(ASTRA) LLC. He has authored or co-authored over 90
refereed papers. He is well known for research on space
weather, ionospheric variability and high latitude
processes, including modeling of the global ionosphere-
thermosphere system in response to geomagnetic storms.
He is PI of the DICE Cubesat mission, and leads several
technology development projects at ASTRA including the
development of software-based receivers, HF sounders,
satellite avionics and ultraviolet remote sensing
instruments. He is a founding member and serves on the Executive Council of the American Commercial Space
Weather Association (ACSWA). He holds a B.Sc. (Hons)
in Physics from Durham University, UK, and a Ph.D. in
Ionospheric Physics from the University of Leicester, UK.
Gary Bust received his Ph.D. in Physics from the
University of Texas at Austin, and is currently a Senior
Research Scientist at Atmospheric and Space Technology
Research Associates. He has worked in the areas of
ionospheric tomographic imaging, data assimilation and
the application of space-based observations to ionospheric
remote sensing for over 17 years. Dr. Bust has authored
over 30 papers on ionospheric imaging and has continued development of the ionospheric imaging algorithm:
“ionospheric data assimilation four-dimensional”
(IDA4D) over the last 17 years.
Adam Reynolds is an engineer at ASTRA. He received
his B.S. in Electrical Engineering from Trinity University
in San Antonio, in 2005. His interests are in both
hardware and software for scientific discovery.
Irfan Azeem is a Senior Engineer at ASTRA. He holds a
B.Eng. (Hons) in Electronics Engineering from Hull
University, UK, and M.S. and Ph.D. degrees in Electrical
Engineering and Space Sciences from the University of Michigan, Ann Arbor. His interests range from GPS to
remote remote sensing of the upper atmosphere. Dr.
Azeem leads research projects in optical remote sensing,
mesospheric dynamics, and instrument development for
space weather/space physics studies.
Rick Wilder is a post-doctoral researcher at ASTRA. His
research interests include GPS, ionospheric radar, and magnetospheric physics. He holds a B.S., M.S. and Ph.D.
in Electrical Engineering from Virginia Tech.
Brady W. O’Hanlon is a graduate student in the School of
Electrical and Computer Engineering at Cornell
University. He received a B.S. in Electrical and
Computer Engineering from Cornell University in 2007.
His interests are in the areas of GNSS technology and
applications, GNSS security, and GNSS as a tool for
space weather research.
Mark L. Psiaki is a Professor in the Sibley School of
Mechanical and Aerospace Engineering. He received a B.A. in Physics and M.A. and Ph.D. degrees in
Mechanical and Aerospace Engineering from Princeton
University. His research interests are in the areas of
GNSS technology and applications, spacecraft attitude
and orbit determination, and general estimation, filtering,
and detection.
Steven Powell is a Senior Engineer with the GPS and
Ionospheric Studies Research Group in the Department of
Electrical and Computer Engineering at Cornell
University. He has been involved with the design,
fabrication, testing, and launch activities of many
scientific experiments that have flown on high altitude balloons, sounding rockets, and small satellites. He has
designed ground-based and space-based custom GPS
receiving systems primarily for scientific applications.
He has M.S. and B.S. degrees in Electrical Engineering
from Cornell University.
Todd E. Humphreys is an assistant professor in the
department of Aerospace Engineering and Engineering
Mechanics at the University of Texas at Austin and
Director of the UT Radionavigation Laboratory. He
received a B.S. and M.S. in Electrical and Computer
Engineering from Utah State University and a Ph.D. in Aerospace Engineering from Cornell University. His
research interests are in estimation and filtering, GNSS
technology, GNSS-based study of the ionosphere and
neutral atmosphere, and GNSS security and integrity.
Jahshan A. Bhatti is pursuing a Ph.D. in the
Department of Aerospace Engineering and Engineering
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Mechanics at the University of Texas at Austin, where he
also received his M.S. and B.S. He is a member of the UT
Radionavigation Laboratory. His research interests are in
the development of small satellites, software-defined
radio applications, space weather, and GNSS security and
integrity.
ABSTRACT
GPS receivers can be used for monitoring space weather
events such as TEC variations and scintillation. This
paper describes the new GPS sensor developed by
ASTRA, Cornell and UT Austin. The receiver is called
“Connected Autonomous Space Environment Sensor
(CASE)”, and represents a revolutionary advance in dual
frequency GPS space-weather monitoring. CASES is a
paperback-novel-sized dual-frequency GPS software
receiver with robust dual-frequency tracking performance,
stand-alone capability, and complete software
upgradability. The receiver tracks L1 and L2 civilian signals (specifically L1 C/A, L2 CL and L2 CM).
The sensor measures and calculates TEC with a relative
accuracy of a few 0.01 TECU at a cadence of up to 1 Hz
(post-processing up to 100 Hz). It measures amplitude
and phase at up to 100 Hz on both L1 and L2-C, for up to
14 satellites in view. It calculates the standard scintillation
severity indicators S4, �0, and ��, and a new index, the
Scintillation Power Ration (SPR), all at a cadence that is
user defined. It is able to track through scintillation with
{S4, �0, amplitude} combinations as severe as {0.8, 0.8
seconds, 43 dB-Hz (nominal)} (i.e., commensurate with vigorous post-sunset equatorial scintillation) with a mean
time between cycle slips of 480 seconds and with a mean
time between frequency-unlock greater than 1 hour.
Other capabilities and options include: Various data
interface solutions; In-receiver and network-wide
calibration of biases, and detection and mitigation of
multipath; Network-wide automated remote configuration
of receivers, quality control, re-processing, archiving and
redistribution of data in real-time; Software products for
data-processing and visualization.
CASES has been designed and developed by the
ionosphere community rather than adapting a commercial receiver. The low price of the sensor means that many
more instruments can be purchased on a fixed budget,
which will lead to new kinds of opportunities for
monitoring and scientific study, including networked
applications. Other potential uses for CASES receivers
include geodetic and seismic monitoring, measurement of
precipitable water vapor in the troposphere at meso-scale
resolution, and educational outreach.
I. INTRODUCTION
Ionospheric weather includes gradients and irregularities
that affect transionospheric UHF and L-band line-of-sight propagation (scintillation) and VHF/HF sky-wave and
scatter propagation. Such disturbances lead to
communication and navigation outages with operational
impacts. These disruptive phenomena occur frequently in
the tropics, less frequently in the polar region, and even
less frequently at mid-latitudes, but they occur
everywhere with serious consequences. Mitigating these impacts through ionospheric specification, now-casting,
and forecasting is a goal now within reach. While
assimilative models have been developed that utilize
ionospheric data, there is insufficient ionospheric data to
adequately specify the global ionosphere. The main
reasons are: (1) existing instruments are inadequate; (2)
existing instruments are too expensive; (3) 70% of the
earth is covered in water, making instrument deployment
difficult. The development of tools to enable “actionable”
ionospheric weather forecasts and specification of
irregularities is the ultimate goal of ASTRA LLC, and
development of the CASES GPS receiver is part of the strategy.
Section II of this paper contains a description of the need
for a hardware platform like CASES, and a brief history
of its development. Section III describes the six salient
features that make this new receiver so popular. Section
IV describes some of the post-processing and user-
interface options available with CASES, and additional
data services available from ASTRA, and Section V
contains conclusions.
II. THE NEED
Space weather refers to conditions in space (the Sun, solar wind, magnetosphere, ionosphere, or thermosphere) that
can influence the performance and reliability of space-
borne and ground-based technological systems.
Ionospheric irregularities at equatorial, auroral, and
middle latitudes constitute a major category of space
weather effects that need to be better characterized and
understood.
Figure 1 shows a mosaic of ultraviolet images from the
nightside ionosphere obtained by the Global Ultra Violet
Imager (GUVI) on NASA’s Thermosphere-Ionosphere-
Mesosphere Energetics and Dynamics (TIMED) satellite.
The image illustrates many important ionospheric features. The geographic equator is in the center of the
figure, with the north pole at the top. The magnetic
equator is not aligned with the geographic equator, and is
shown as the wavy green line at low latitudes. The
brightest portions of the UV image occur on either side of
the magnetic equator, and are caused by the large electron
density in the Appleton Anomalies. The brightness of the
images falls off towards the equator and middle latitudes
because the electron concentrations are lower. Brighter
UV emissions are observed in both polar regions, due to
the effects of auroral particle precipitation.
At low latitudes, each image shows extended airglow
depletions with braided irregularities. The airglow
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Table 1. Performance characteristics of the CASES space weather monitor
FeatureCASES Receiver
L1C/A & L2-C carrier phase and amplitude data Yes, 14 channels each, at 100 Hz (e.g. I’s and Q’s)
L1 C/A & L2-C pseudorange data Yes, 14 channels each, up to 1 Hz
TEC Output Yes, up to 1 Hz (relative accuracy ~0.01 TECU)
Scintillating parameter output S4, s�, �o, Scint. Power Ratio, (50 sec, 100 sec)
Mean time between cycle slips for {S4 =
0.8,�o = 0.8 sec, C/No = 43 dB-Hz}
480 seconds
Tra
ck
ing
ro
bu
stn
ess
L2-C tracking threshold 25 dB-Hz (expected)
Reconfigurability Complete sensor reconfigurability downstream of
ADC:
correlation/accumulation, tracking loops,
computation of observables
Stream digitized RF front-end samples to disk Yes
Communication Ports Ethernet, WiFi, RS-232
USB host (e.g hard drive)
Push data to external FTP site Yes
Single Board Computer peripherals Variety of ports and protocols can accept external
sensor data/integration (e.g. magnetometer, humidity, etc)
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III. SIX SALIENT FEATURES OF THE CASES
RECEIVER
The CASES space weather sensor is a low-cost, open-
architecture, science-grade, dual-frequency GPS receiver.
It lies at the heart of the ionospheric monitoring system
being developed by ASTRA. The new CASES sensor combines several radical innovations that make it well
suited for space weather diagnosis and provide
advantages over competing GPS sensors:
Feature #1: Software Radio Technology. Whereas
traditional GPS receivers depend on one or more special-
purpose ASIC chips, the CASES sensor implements all
digital signal processing functions (i.e., those downstream
of its analog RF front end) -- from correlation to
navigation solution -- in a general-purpose DSP. The
fourfold benefits of the software radio approach are:
• flexibility to accommodate specialized signal
processing schemes like the scintillation-robust tracking loops discussed below;
• remote reconfigurability; i.e., an entirely new
receiver personality can be downloaded from a
remote location;
• full control over receiver behavior, products,
and cadences; and
• reduced cost, because the receiver uses a
mass-produced COTS DSP chip in place of special-
purpose GPS chips used by other manufacturers.
Feature #2: Use of the new L2C signal. The sensor is
capable of receiving and processing not only the usual C/A-code signal on the GPS L1 frequency, but also the
new CM (civil medium length) and CL (civil long length)
codes now being broadcast on the GPS L2 frequency.
The new signals, known jointly as L2C, are the first of a
new set of signals that the GPS Wing of the U.S. Air
Force plans to roll out as part of a larger GPS
modernization effort. The new L2C signals on Block IIR-
M and IIF satellites make possible the creation of more
robust, less complex, and yet more precise GPS Total
Electron Content receivers and scintillation monitors.
Since the L2C signal began broadcasting, several L2C-
capable GPS receivers have emerged in the marketplace, but these are expensive (> $10k) and are not particularly
well suited for space weather monitoring. The continued
development of GPS software receivers at Cornell,
together with Cornell’s recent demonstration of software
receivers operating on DSP chips6, have opened the door
for inexpensive GPS software receivers that rapidly
exploit the new signals and that are easily configured in
networks.
GPS receivers such as the CASES sensor that are capable
of tracking the new L2C signal enjoy three advantages
compared with traditional dual-frequency civilian receivers:
• First, the absence of data bit modulation on the L2
CL code permits tracking with a non-squaring-type
phase-locked loop (PLL) instead of the standard
squaring-type PLL used to recover carrier phase in the
presence of zero-mean binary phase modulation. This
leads to a significant 6-dB improvement in the
receiver's tracking threshold. The full 6-dB benefit
relative to L1 C/A tracking will not be immediately realized because the native L2 signal power is 3 dB
lower than that of L1, but it is likely that the L2 signal
power will be increased as part of the GPS
modernization effort.
• The second advantage of tracking the new L2C
signal also leads to an improvement in the tracking
threshold, but for a different reason. Until recently
only an encrypted military signal was available on L2,
and the only option for extracting dual-frequency
observables in a civilian receiver was to employ one of
several proprietary signal processing techniques to
recover the L2 carrier. Such techniques, while impressive in their ingenuity, pay a severe price in
squaring loss for their lack of knowledge of the
encrypted military code. For example, the state-of-the-
art technique for L2 carrier recovery suffers a squaring
loss of 11 dB at a nominal carrier-to-noise ratio (C/N0)
of 39 dB-Hz and a loss of 19-dB at C/N0 = 30 dB-Hz.
Such a degraded tracking threshold makes traditional
civilian tracking on L2 extremely fragile, leading to
repeated cycle slips and even frequency-unlock during
weak signal tracking or strong ionospheric scintillation.
In contrast, L2C-capable GPS receivers can, when tracking the L2CL code, avoid both data-bit- and
encryption-induced squaring losses, with a better than
11-dB improvement in signal tracking threshold.
• The third advantage of an L2C-capable GPS
receiver is cost, as discussed below.
Feature #3: Tracking Through Weak Signals. The third
feature of the CASES sensor that sets it apart from
standard dual-frequency GPS receivers is the
incorporation of specialized tracking loops designed for
operation in both weak-signal and scintillating
environments. Over the past few years, several of the
CASES partners have been engaged in a focused research effort to study PLL behavior in weak signal and
scintillating conditions7,8,9. The research has identified
tracking loop architectures and parameters suited for
weak-signal tracking and others suited for tracking during
scintillation. Tracking techniques that adapt to the signal
conditions and use data-bit or parity aiding (when
tracking data-bit-modulated signals) offer the most
promise9. The CASES team has developed a laboratory
facility for testing GPS receivers under realistic
scintillation conditions10. This facility, combined with the
CASES sensor’s flexible open software architecture, has permitted tracking loop strategies to be tested and tuned
for robustness.
Other commercial receivers, while effective in avoiding
total loss of lock (frequency unlock) because of their
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IV. VALIDATION, POST-PROCESSING AND
USER-INTERFACE OPTIONS
The testing and validation of the CASES receiver is
described in detail in an accompanying paper in this
issue5. The receiver has been tested and validated using
both real and simulated data in an effort to confirm the expected performance, including the precision with which
it can produce observables such as phase and
pseudorange. Tests indicate the CASES onboard
processing delivers an RMS error for a single receiver of
0.35 meters in pseudorange.
In the laboratory, the data bit prediction tracking
algorithms were tested using data from a Spirent
simulator for cases of strong scintillation. Figure 4 depicts
the phase error measured by CASES without the use of
data bit prediction, and Figure 5 shows the result when
the algorithm was used. Both tests were performed under
simulated heavy scintillation defined by S4 = 0.8, �0 = 0.8 sec, C/N0 = 43 dB-Hz. Using the data bit prediction
algorithm, the mean time between cycle slips was 480
sec. This level of performance exceeded the design
requirements of CASES by a factor of two. These results
are discussed in more detail in an accompanying paper5.
Fig 4. Phase error measured by CASES without the data
bit tracking algorithm
CASES receivers have operated reliably for many months
at ASTRA’s headquarters in Boulder, CO, in laboratory
environments in Austin, TX, and at Cornell. CASES
receivers have also operated successfully under harsher conditions in a number of geophysically interesting
locations during several field campaigns. A receiver has
operated reliably in Jicamarca, Peru for almost a year, and
seven receivers operated reliably in Peru during a
campaign lasting several weeks. Two CASES receivers
operated reliably for several months at South Pole,
Antarctica, and two more will be deployed in Antarctica
this year.
Fig 5. Phase error measured by CASES with the data bit
tracking algorithm activated
In addition to the on-board processing, ASTRA has also
developed a suite of post-processing software and
services that can be made available to the user. Table 2indicates some of the post-processing options available
with CASES. These range from a simple user interface,
to a sophisticated system that integrates data from
multiple nearby receivers and applies various quality
control algorithms to the data. ASTRA also has the
ability to ingest multiple ionospheric data sets into an
ionospheric assimilation algorithm known as IDA4D
(Ionospheric Data Assimilation in 4-D)12,13,14, and to
produce HF propagation maps from the resultant regional
and global ionospheric specification.
The user interface developed by ASTRA means that the CASES receiver is useful ‘straight out of the box’. The
user interface software includes plotting options like maps
to indicate where various CASES receivers are located,
and options to plot various parameters being computed
locally by the receiver. For example, Figure 6 depicts the
variation of the S4 index computed from GPS signal
collected from several different GPS satellites over a
period of several hours by a CASES receiver in Austin,
TX. At low elevations, multipath masquerades as
scintillation and raises the S4 level, and the user should
activate an elevation mask to remove these erroneous S4
values. The particular antenna used, and the multipath environment, are both significant factors in obtaining high
accuracy with any GPS receiver.
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Table 2. Postprocessing software options available with
CASES receiver
Fig 6. ASTRA’s User Interface software includes plotting
routines that provide immediate visualization of the data
coming from the CASES receiver, together with a number
of receiver control options, and the ability to upload new
software or configuration controls to the receiver.
The amount of data being produced by the CASES
receiver can be quite large, especially when being
recorded at 100 Hz. CASES data rates can vary widely
depending on how the user sets the flags in the
configuration file.
The low rate data requires approximately 63 bytes per
sample. If the user streams low rate data at 1 second per
sample, that comes out to 63 B/s per channel. Typical
satellite coverage gives about 12 channels in use at any
given time, which gives a total requirement of 750 B/s, or
about 65 MB/day. Lowering the data rate to 5 seconds
per sample will drop this to 150 B/s and 13 MB/day. The
user can also set a limit on the number of channels that
can be tracked at any given time, anywhere from 1
channel to the default of 22 channels. Our system in
Antarctica is set up to track a maximum of 4 channels at
any given time, and send data at 15 seconds per sample, giving it a typical daily requirement of around 1 MB,
depending on satellite coverage.
The high rate data uses the most bandwidth, as it requires
approximately 25 bytes per sample and is sent at 100 Hz,
giving a bandwidth requirement of about 2.5 kB/s per
channel. For typical 12 channel coverage, this gives a
total requirement of 30 kB/s, or about 2.6 GB/day. Again,
this can be trimmed by setting a limit on the number of
channels that send high rate data, and/or activating a
scintillation monitor to selectively send high rate data
depending on measured scintillation activity. Our system in Antarctica is set to send high rate data for a maximum
of 4 channels at any given time, using the scintillation
monitor with a scintillation power ratio threshold of -20
dB. This produces roughly 5 MB/day in high rate data.
It should be noted that the typical 12 channel requirement
is based on the current L2C coverage and about 10
satellites in view (10 satellites in view, 2 of which are
broadcasting L2C, giving 10 L1 + 2 L2C = 12 channels
total). As the L2C coverage grows, so the CASES
bandwidth requirements will grow. The accompanying
paper by O’Hanlon et al5. describes a triggering technique
to reduce the amount of unwanted data.
In order to help our customers deal with these challenges,
ASTRA offers three different levels of service with the
CASES receiver, as shown in Table 3. These range from
Level-1, where the user is completely independent,
manages their own data, and uses only the data extraction
software developed by ASTRA; to Level-3, where
ASTRA manages the user’s data,
including archiving, processing and quality control, and
the user accesses their data through the ASTRA computer
network. In between, at Level-2, the user is licensed to
use ASTRA’s post-processing and visualization software but remains somewhat independent.
Post-Processing
Module
Task
User Interface Provides user with a graphical
interface to control and update
a network of receivers, and
visualize data
Single Unit QC Cycle slip correction, phase
debiasing, TEC debiasing
Multi Unit QC Compare data from multiple
nearby receivers to remove
outliers
Scintillation Compute scintillation
parameters (S4, Sigma Phi,
ROTI)
System Integration Combine user interface with
QC and scintillation modules
HF Propagation Predicts line of sight RF
propagation using model
ionosphere
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Table 3. CASES service options available from ASTRA
V. CONCLUSIONS
The CASES software-defined dual-frequency GPS
receiver has been developed as a space weather monitor.In addition to the navigation solution obtained by other
GPS receivers, the CASES receiver also measures
ionospheric TEC and scintillation with high accuracy and
reliability. The receiver outputs phase and amplitude
information, together with timing and ephemeris on a 100
Hz cadence. In addition, the receiver computes
ionospheric TEC and scintillation parameters on a
cadence that is user selectable (typically 30 – 60 sec). The
scintillation parameters delivered by CASES include S4,
�0, ��, and the Scintillation Power Ratio. The software
receiver uses a number of novel algorithms for on-board processing of the GPS signals on L1C/A and L2C. In
addition, post-processing of the 100 Hz data provides
additional opportunities for quality control
The CASES receiver is being commercialized by ASTRA
LLC (www.astraspace.net) of Boulder, CO and is
available off-the-shelf at short notice, in two different
form factors, as shown in Figure 7. The original CASES
receiver is approximately the size of a large paperback
book with a volume of 130 cubic inches. However, a
second form factor is available, a modified stack having a
volume of only 40 cubic inches. This smaller form factor
is less than 4” on each side, making it suitable for use in a Cubesat. The approximate mass of the three boards in
CASES are 50 grams, 60 grams, and 75 grams for the
RFE, DSP, and SBC boards, respectively. A space-based
receiver will utilize only the RFE and DSP boards (110
grams total, approx). The current version of CASES
consumes about 6.5W of power, however a lower-power
version is under development at ASTRA.
ACKNOWLEDGEMENTS
The authors would like to thank the Air Force Office of
Scientific Research for providing partial funding for this project through an STTR award to ASTRA, LLC of
Boulder, Colorado.
Fig 7. Two different form factors are available for the
CASES receiver. The smaller one (top) is designed to fit
inside a Cubesat.
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Product
Options
Includes
Level 1 CASES Receiver, ASCII data extraction
software
Level 2 Level 1 + ASTRA post-processing and data
visualization software
Level 3 Level 1 + Access to ASTRA generated
quality controlled, post-processed data and
visualization for customer’s CASES
receivers
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[9] Humphreys, T.E., M.L. Psiaki, and P.M. Kintner, Jr.,
"Modeling the effects of ionospheric scintillation on
GPS carrier phase tracking," IEEE Transactions on
Aerospace and Electronic Systems, October 2010.
[10] Humphreys, T.E., M.L. Psiaki, J.C. Hinks, B.W.
O’Hanlon and P.M. Kintner, Jr., "Simulating Ionosphere-Induced Scintillation for Testing GPS
Receiver Phase Tracking Loops,"IEEE Journal of
Selected Topics in Signal Processing, Vol. 3, No. 4,
2009
[11] Mohuiddin, S., T.E. Humphreys, M.L. Psiaki, "A
Technique for Determining the Carrier Phase
Differences between Independent GPS Receivers
during Scintillation," Proc. ION GNSS, The Institute
of Navigation, Fort Worth, TX, 2007.
[12] Bust, G.S., and G. Crowley (2008), “Mapping the
time-varying distribution of highaltitude plasma
during storms”, in Midlatitude Ionospheric Dynamics and Disturbances, Geophys. Monogr. Ser., vol. 181,
edited by P.M. Kintner, A.J. Coster, T. Fuller Rowell,
A.J. Mannucci, M. Mendillo and R. Heelis, pp. 91-
98, AGU, Washington, D.C.
[13] Bust, G. S., and G. Crowley (2007), “Tracking of
Polar Cap Ionospheric Patches using Data
Assimilation”, J. Geophys. Res., 112, A05307,
doi:10.1029/2005JA011597.
[14] Bust, G. S., G. Crowley, T. W. Garner, T. L.
Gaussiran II, R. W. Meggs, C. N. Mitchell, P. S. J.
Spencer, P. Yin, and B. Zapfe (2007), Four Dimensional GPS Imaging of Space-Weather Storms,
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doi:10.1029/2006SW000237.
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